Demand for interim rental Housing Board (HDB) flats almost doubled last year as more families were affected by Build-To-Order (BTO) construction delays because of the Covid-19 pandemic.

The board received 2,350 applications under the Parenthood Provisional Housing Scheme (PPHS) compared with 1,370 in 2019, said the Ministry of National Development (MND) in a written parliamentary reply on Tuesday.

But there were only 160 available flats last year.

The PPHS provides help to households awaiting the completion of their new flats, with units allocated by ballot.

Rents are $400 for a two-room flat in Marsiling, $600 for a three-room type in Hougang and $1,500 for a four-room unit in Tiong Bahru.

While application numbers have increased, the MND said not all households may be in urgent need of temporary housing.

Some may have sought other options as about 40 per cent of successful applicants did not turn up for flat selection.

Food technologist Jowee Ng, 29, and his wife Joslyn Chua, 26, an engineer, consider themselves to be "one of the lucky ones" after they landed a unit on their fifth try.

"Honestly, we were already thinking of giving up if we failed on the fifth try and just renting a flat on the open market because we've seen how high the PPHS application rates were. We were quite surprised we got it," said Mr Ng.

They moved into the two-room flat in Canberra last month and spent around $6,000 on furnishings. The monthly rent is $400, excluding utility bills. They plan to live there until their BTO unit in Tengah is ready in 2023.

"We were previously living separately and met only on weekends. Now that we're living together, we have a better understanding of each other's habits at home and even developed new hobbies, such as gardening together," said Mr Ng. He said of the flat: "It's small but cosy and it feels like home. It's good practice before the real deal comes along."

Married couples made up 66 per cent of the 2,350 applications last year while couples applying under the fiance/fiancee scheme comprised 31 per cent.

The remaining 3 per cent were applicants who are divorced or widowed with children.

The supply of PPHS flats is limited and depends on factors such as availability of vacated blocks that are not immediately needed for redevelopment.

There are about 110 two-room, 570 three-room, and 60 four-room flats under the PPHS across Singapore.

The HDB said it is working towards a more stable supply and wider options for families needing temporary housing.

Construction delays faced by BTO developments have increased the demand for such housing.

About 85 per cent of the 89 ongoing BTO projects are around six to nine months behind schedule, affecting about 43,000 households.

These projects are now expected to be delayed by a further three months as bans on new arrivals from countries such as India and Bangladesh have exacerbated the manpower crunch in the construction sector.

Operations administrator Siti Amirah Ramli, 26, and her operations technician fiance Amir Nurhasan Kamarudin, 26, plan to apply for a PPHS unit in July before their October wedding but are concerned that they will miss out.

Couples applying under the fiance/fiancee scheme must submit their marriage certificate to the HDB within three months of taking possession of a PPHS flat.

The couple's BTO unit in Tengah will be ready only at the end of next year. She said they are thinking about a Plan B if their application falls through. "Rent on the open market is too costly for us to manage and we also wouldn't want to spend so much just to fund a temporary home.

"We could rent a single room, instead of a whole unit, but this means living with strangers, so we are not keen," she added.

"Perhaps we'll live with relatives or live separately for some time."